1. Field of the Invention
The present invention relates to hexagonal ferrite magnetic powder and to a method of manufacturing the same. More particularly, the present invention relates to hexagonal ferrite magnetic powder exhibiting good magnetic characteristics and to a method of manufacturing the same.
Still further, the present invention relates to a magnetic recording medium comprising hexagonal ferrite magnetic powder obtained by the above manufacturing method in a magnetic layer, and to a method of manufacturing the same.
2. Discussion of the Background
Conventionally, ferromagnetic metal magnetic particles have been primarily employed in the magnetic layer of magnetic recording media for high-density recording. Ferromagnetic metal magnetic particles are acicular particles comprised chiefly of iron, and have been employed in magnetic recording media for various applications requiring particle size reduction and high coercive force for high density recording.
With an increase in the quantity of information being recorded has come the requirement of recording at ever higher densities on magnetic recording media. However, in recording at higher densities, limits to improvement of the ferromagnetic metal magnetic particles have begun to appear. By contrast, hexagonal ferrite magnetic particles can exhibit coercive force of a degree found in permanent magnetic materials. The magnetic anisotropy that is the basis of coercive force is derived from a crystalline structure, so a high coercive force can be maintained even when the size of the particles is reduced. A magnetic recording medium with a magnetic layer in which hexagonal ferrite magnetic particles are employed will exhibit good high density characteristics due to its vertical component. Thus, hexagonal ferrite magnetic particles are a ferromagnetic material that is suited to achieving higher densities.
Known methods of manufacturing hexagonal ferrite magnetic powder include the glass crystallization method, the hydrothermal synthesis method, and the coprecipitation method. The glass crystallization method is said to be a good method of manufacturing hexagonal ferrite for use in magnetic recording media because magnetic powder having the desired suitability to size reduction and single particle dispersibility in magnetic recording media can be obtained. As a result, various methods of manufacturing hexagonal ferrite magnetic powder by the glass crystallization method have been studied (for example, see Japanese Unexamined Patent Publication (KOKAI) No. 2003-162809, which is expressly incorporated herein by reference in its entirety).
The process of manufacturing hexagonal ferrite magnetic powder by the glass crystallization method generally comprises the following steps:
(1) Melting a starting material mixture comprising hexagonal ferrite-forming components and glass-forming components to obtain a melt;
(2) Rapidly cooling the melt to obtain a solidified product (amorphous material);
(3) Heat treating the solidified to cause hexagonal ferrite magnetic particles (referred to as “hexagonal ferrite particles” or simply “particles”, hereinafter) and a glass component to precipitate out; and
(4) Subjecting the heat-treated product to an acid treatment and a cleaning treatment to dissolve away the glass component, and collecting the hexagonal ferrite magnetic particles.
The particle diameter of the hexagonal ferrite particle obtained by the above process can be controlled by means of the heating conditions (heating temperature and duration) in step (3) above. The heating conditions are determined so that particles of the desired size precipitate out. However, a size distribution is present in the particles that precipitate out, and it is extremely difficult to conduct the process in a manner that avoids forming ultrafine particles that are much smaller than the desired particle diameter. However, these ultrafine particles include components that become superparamagnetic and do not contribute to magnetic recording and components that are affected by thermal fluctuation and destabilize magnetization, and are thus desirably removed before manufacturing a magnetic recording medium. In this regard, Japanese Unexamined Patent Publication (KOKAI) No. 2003-162809 proposes subjecting hexagonal ferrite magnetic powder that has been produced by the glass crystallization method to wet sorting with an elutriation apparatus to remove the fine particles. However, it is difficult to sort to a high degree with an elutriation apparatus the primary particles of a nano order of magnitude that are employed in magnetic recording media for high-density recording.
As set forth above, there is need for a new sorting means that makes it possible to selectively remove ultrafine particles from hexagonal ferrite magnetic powder obtained by the glass crystallization method to obtain a hexagonal ferrite magnetic powder that can exhibit good magnetic characteristics in magnetic recording media for high-density recording.